On the eve of the arraignment he slammed the case as proof of the "corruption, scandal, & failure" of the US under Joe Biden's presidency.

Security is being ramped up in Washington DC for Thursday's hearing.

Mr Trump already faces two other criminal cases as he campaigns for the White House next year.

But the election case is widely seen as the most serious one of all.

In an all-capital-letters post on his Truth Social platform on Wednesday, Mr Trump, who was at his Bedminster, New Jersey, golf club, thanked his followers and said: "I never had so much support on anything before."

In other posts he attacked rival Republican presidential hopefuls, including his former Vice-President Mike Pence and Florida Governor Ron DeSantis.

He repeated his argument that Mr Pence had had the legal authority to stop Congress from certifying Mr Biden's election victory on 6 January 2021, proceedings that were disrupted as Trump supporters rioted at the US Capitol.

Mr Trump, 77, is due to appear at a federal courthouse in Washington DC on Thursday at 16:00 EDT (20:00 GMT). He is expected to plead not guilty. Although he has the option of appearing remotely via video feed, it is understood he will attend in person.

Mr Trump has visited the city only once since leaving the White House. A queue was already forming on Wednesday evening outside the building.

In the indictment unsealed on Tuesday, he faces four counts, including conspiracy to defraud the US, tampering with a witness and conspiracy against the rights of citizens.

The deprivation of rights statute was enacted in the aftermath of the US Civil War to provide legal protections for freed slaves as they integrated into society.

The charge was key to a 1967 trial of Ku Klux Klan members that inspired the 1988 film Mississippi Burning. Legal analysts say it has been included in the case against Mr Trump because his alleged efforts to subvert the vote targeted urban areas with large populations of African-American voters.

Mr Trump's lawyers have hinted at their defence strategy.

Attorney John Lauro appeared on NBC's Today show on Wednesday and said he planned to argue that Mr Trump was protected by the right to free speech enshrined in the First Amendment of the US Constitution.

"[The indictment] is criminalising speech," Mr Lauro said.

Mr Trump's legal team are also resisting the prosecutors' desire for a speedy trial - saying they need time to organise their client's defence.

A spokesman for the US Marshals Service, a federal law enforcement agency that guards courts, told Reuters news agency Mr Trump would be fingerprinted and asked to provide basic information, such as his date of birth and Social Security ID number.

Several other suspects accused of involvement in the US Capitol riot have had hearings that were scheduled for Thursday postponed.

Federal agencies and local police have been stepping up security measures in the nation's capital ahead of Mr Trump's trip.

The Secret Service, which provides protection to presidents and ex-presidents, released a statement warning the public of "short-term traffic implications" in central Washington DC.

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Secret Service spokesman Anthony Guglielmi said the agency would "ensure the highest levels of safety and security for the former president, while minimizing disruptions to the normal court process".

On Wednesday, an apparent hoax 911 call about an active shooting at the Capitol triggered a lockdown in three Senate office buildings and a major police emergency response.

Speaking to reporters during the alert, US Capitol Police Chief Tom Manger said the police force that battled rioters two-and-a-half years ago was well-trained and prepared for active-shooter drills.

"We're prepared for anything that may happen tomorrow," he said, referring to Mr Trump's court appearance.

Mr Trump is currently the clear front-runner in the Republican party's contest to pick its next presidential candidate.

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Congressional Republicans have been rallying round him, arguing that the latest indictment shows the US has become a "banana republic" and echoing the former president's claim that the prosecutions amount to election interference.

Mr Pence, who has been struggling to gain traction in the 2024 White House race, maintained on Wednesday during a campaign stop at the Indiana State Fair that he had "done his duty" on January 6 2021.

"Sadly the president was surrounded by a group of crackpot lawyers that kept telling him what his itching ears wanted to hear," he said. "The president ultimately continued to demand that I choose him over the Constitution."

The 45-page election-related indictment against Mr Trump is based partly on contemporaneous notes that Mr Pence kept of their conversations in the days leading up to US Capitol riot.

Mr Trump has already been charged in two other cases: with mishandling classified files and falsifying business records to cover up a hush-money payment to a porn star.

Prosecutors in Georgia may bring a criminal case against Mr Trump this month over alleged efforts to subvert the 2020 election result in that state.

By Max Matza & Sarah Smith